Directional solidification is a method of casting whereby a solidification interface is caused to move progressively through a mold filled with molten metal. Both columnar grain castings, such as disclosed in U.S. Pat. No. 3,260,505 to VerSnyder and single crystal castings, such as those disclosed in U.S. Pat. No. 3,494,709 to Piearcey, are usefully made by directional solidification. In the most common industrial practice, articles are formed by making a ceramic mold in the shape desired, placing the mold within a furnace, and causing the molten metal poured into the mold to cool progressively from the bottom of the mold to the top.
In certain circumstances it is desired that the article have a very particular crystallographic structure. For example, a single crystal casting may be sought where there is a particular orientation of the crystallographic axes with respect to the x, y and z axes of the article. In other instances, it may be desired that an article have different structures in different parts of the casting. To achieve such results, it is common to use a metal seed, placed within the mold prior to the introduction of the molten metal. When the molten metal is poured into the mold, the seed is caused to partially melt where it is contacted by the molten metal. Thereafter, progressive cooling of the casting causes metal to solidify epitaxially from the portion of the seed which was not melted.
It is obvious that a seed cannot be entirely melted, and that it must be somewhat melted, to carry out the objects of epitaxial solidification. However, in commercial and experimental casting of nickel superalloy articles, it is observed that even when casting takes place within the foregoing obvious limitations, the resultant cast articles do not always have the crystallographic structure which is desired and expected as a result of epitaxial growth from the seed. Consequently, research work has been undertaken to discover a solution to the problem of improving seeded casting yields.